1. Field of the Invention
The present invention relates to a traveling speed control system for a work vehicle equipped with an engine having a variable throttle opening and a hydrostatic transmission (hereinafter, referred to as “HST”) whose output/input speed ratio is increased in correspondence to an increase in a set traveling speed so as to increase the traveling speed of the work vehicle.
Incidentally, in the major cases in the following description, the “traveling speed control system” for a work vehicle is referred to as “vehicle speed control system”, and the “traveling speed” of the work vehicle is referred to as “vehicle speed”.
2. Related Art
There is a well-known conventional traveling speed control system for a work vehicle equipped with an engine and an HST, as disclosed by JP 2004-255919 A. This vehicle speed control system increases the vehicle speed by increasing a throttle opening of the engine and an output/input speed ratio of the HST in correspondence to an increase of vehicle speed set by an acceleration manipulator (hereinafter referred to as “accelerator”). The throttle opening is at its minimum during idling of the engine before start of the vehicle. An increase rate of the throttle opening during, increase of vehicle speed from 0 (the idling of the engine) to an intermediate speed is smaller than that during increase of vehicle speed from the intermediate speed to the maximum speed, and meanwhile, an increase rate of the output/input speed ratio during increase of vehicle speed from 0 to an intermediate speed is smaller than that during increase of vehicle speed from the intermediate speed to the maximum speed. The throttle opening increasing at the large increase rate ensures a smooth start of the vehicle and stable low speed travel of the vehicle.
However, the disclosed conventional vehicle speed control system postulates that the work vehicle is not heavily loaded during the engine idling before the start of traveling of the work vehicle, i.e., before the accelerator is manipulated to set a vehicle speed larger than 0. If the work vehicle is adapted to be heavily loaded with a work device, e.g., a cultivator, a mower unit or a pest control machine, during the engine idling before traveling start of the vehicle or during a low speed working travel of the vehicle on a rough field, the engine power may be insufficient to start the vehicle, thereby causing unstable start or working travel of the vehicle or undesired stopping of the engine.
On the other hand, if a work vehicle adapted to receive such a heavy load travels normally with no heavy load, the increase of the throttle opening at the large increase rate for starting the vehicle causes excessively high-speed rotation of the engine, shortening the life of the engine, and causes waste consumption of fuel, increasing costs.